Hot melt adhesives are adhesives which are capable of efficiently adhering a variety of products due to their feature of solidifying in a short time period, and are highly safe to human body since the adhesives do not require a solvent. Therefore, the hot melt adhesives are utilized in various fields. For example, in the manufacture of hygiene products such as paper diapers and sanitary products, hot melt adhesives are routinely used to bond members that constitute those products. Furthermore, hot melt adhesives are also routinely used as adhesives that are used in various adhesive tapes, labels for office use or industrial use, and the like.
Various polymers are used as base polymers that constitute hot melt adhesives. As the hot melt adhesives used for the manufacture of hygiene products, hot melt adhesives which use aromatic vinyl-conjugated diene-aromatic vinyl block copolymers such as a styrene-isoprene-styrene block copolymer (SIS) or a styrene-butadiene-styrene block copolymer (SBS) as base polymers, are used to a large extent since these adhesives have excellent adhesiveness to polyolefins that are frequently used as a material for the members of hygiene products.
However, hot melt adhesives that are used for the manufacture of hygiene products are required to have various performances, and among them, performances such as easy appliability at a relatively low temperature, a long open time after coating, and excellent holding power or tackiness are strongly demanded. In order to achieve such demanded performances, extensive investigations have been conducted on hot melt adhesives which use aromatic vinyl-conjugated diene-aromatic vinyl block copolymers as base polymers.
For example, Patent Literature 1 discloses that a hot melt adhesive which has excellent low temperature adhesive force, has low melt viscosity, and has high adhesive force and high tack force is obtained by constructing a hot melt adhesive using an aromatic vinyl-conjugated diene-aromatic vinyl block copolymer having two aromatic vinyl blocks with different molecular weights. The hot melt adhesive described in this Patent Literature 1 has low viscosity at 160° C., but the viscosity rapidly increases at temperatures lower than 160° C. Accordingly, the hot melt adhesive has a problem that application at lower temperature is difficult, and the open time after application is not sufficient.
Furthermore, Patent Literature 2 discloses that an elastomer composition containing a branched aromatic vinyl-isoprene block copolymer, a linear aromatic vinyl-isoprene block copolymer and polyisoprene, has satisfactory holding power and initial adhesive force (tackiness) and is used as a hot melt adhesive having low viscosity at a relatively low temperature. When this elastomer composition described in Patent Literature 2 is used, it is possible to lower the viscosity at a relatively low temperature, but it cannot be said that the extent of improvement thereon is sufficient, and a new improvement is desired.